Piston



FIG. 1 is a perspective view of an embodiment of a piston showing our new design;

FIG. 2 is a top plan view thereof;

FIG. 3 is a side elevation view thereof, the opposing side elevation view being a mirror image;

FIG. 4 is a bottom plan view thereof; and,

FIG. 5 is a cross-sectional side elevation view thereof taken along line 5-5 of FIG. 3. 

We claim the ornamental design for a piston, as shown and described. 